In many parts of the world, heating and cooking are performed using combustible biomaterial as a fuel source. Combustion with this type of fuel often is incomplete leading to production of poisonous gases, especially carbon monoxide. Within a living or enclosed space, use of biomaterials carbon monoxide may build-up causing sickness or death.
Carbon monoxide (CO) is a colorless, odorless, tasteless toxic gas produced by incomplete combustion in fuel-burning. CO poisoning may result in headaches, nausea, dizziness, or confusion. Left undetected, CO exposure can be fatal, and in the United States alone, accidental CO poisoning results in about 15,000 ER visits a year.
Because carbon monoxide is a byproduct of incomplete combustion, procedures that enhance combustion will reduce the production of carbon monoxide. Those of skill in the art will understand that enhancing combustion may generally be accomplished in three ways—by increasing the duration of combustion, raising the temperature at which combustion takes place, or optimizing the mixing of oxygen and fuel. Another contributor to incomplete combustion may be the presence of a heat sink that may quench combustion. In general, it is easier to control these factors when a gaseous fuel is burned as opposed to a solid fuel. Thus, in developed countries, solid fuel has been largely replaced by gaseous fuels for household use. But, as is evident from the carbon monoxide poisoning statistics presented above, even in the United States, improperly maintained natural gas or propane burners may produce significant amounts of carbon monoxide.
Carbon monoxide may be produced by combustion even under controlled conditions using modern appliances. For this reason, modern must be carefully engineered to properly mix air with gas and modern appliances are generally vented to allow exhaust to be directed out of the house. In contrast, in other countries, it is not uncommon for households to employ unvented, solid fuel biomass stoves for heating and cooking. Use of biomass creates a significant risk if the stove is used within the living quarters or an enclosed space.
Outside the United States, the predominant combustible material for household energy production come from solid fuels such as biomaterial (for example, without wishing to be limited, pelletized or compressed waste or wood, wood chips, coal, dung or other organic materials such as twigs, grasses, or rice husks). For example, it is estimated that over 70% of African households and 80% of Chinese households burn solid fuels for domestic energy needs. As described above, when solid fuel, especially wood, is burned in confined or poorly ventilated spaces, carbon monoxide levels may build to dangerous levels. It has been estimated that between 1.5 and 2 million people die each year as a result of exposure to indoor air pollution resulting from the use of solid fuels.
Poverty is one of the largest contributing factors to the use of solid biomaterial as a fuel source. For example, studies have shown that per capita gross national product (GNP) is closely correlated with dependence on biomass: countries with lower per capita GNP tend to rely on traditional fuel sources far more than countries with higher GNP. Thus, any solution to the problem of indoor air pollution from the combustion of solid fuels must be both cost effective and must not dramatically impact traditional behavior.
Various stove designs are available that may lessen the risk of using biomass for heating or cooking indoors. These stoves attempt to increase stove efficiency, and thus decrease pollution. Some stoves may be constructed of traditional materials such as brick, stone, or ceramics. Other stoves may be constructed of metal. Some stoves are designed to be constructed with either traditional or modern materials, such as, for example without limitation, “rocket” stoves. Rocket stoves employ an “L” design to control the combustion of fuel and mixing of air. In many rocket stoves, fuel, for example twigs, is slowly introduced to the combustion chamber at the bottom of the L. This slow addition of fuel helps to limit the rate of combustion by confining burning to the tips of the sticks. Rocket stove design may include insulation of the chimney to decrease quenching of combustion by cooler surfaces. Some stoves may be designed with a constant radius for both the upper and lower combustion chamber. While rocket stoves may be designed to control air flow passively, other stove designs use electric fans to force air through.
Gasification stove design may rely on passive air flow but more often employs forced air from electric fans to increase stove efficiency. Gasification stoves, (variously known as fan-stoves, semi-gasification stoves, etc.) offer an alternative to traditional stove designs. Gasification stoves replace direct combustion of biomass fuel with techniques that release volatile gases, which are then ignited separately. Gasification is a process that converts carbon containing materials, such as, for example without limitation, coal, petroleum, biomaterial, or biomass, into carbon monoxide and hydrogen by reacting the raw material at high temperatures with a controlled amount of oxygen and/or steam. The resulting gas mixture is itself a fuel and can be combusted. This process may reduce pollution by reducing incomplete combustion and the amount of material needed to fuel the stove.
Gasification techniques are potentially more efficient than direct combustion of the original fuel because it can be combusted at higher temperatures. In addition, the high-temperature combustion may refine out more corrosive elements such as chloride and potassium, allowing relatively cleaner combustion in some cases as well as higher efficiency. However, gasification stoves may be more difficult to construct than some other types of stoves, and therefore more expensive to produce.
To reduce costs of a solid fuel stove for household use and make it accessible to low income persons, requires that the materials used in its construction be inexpensive and that the manufacturing process be efficient and low cost. This is difficult because the combustion environment associated with the use of solid fuels is extreme, both in temperature and corrosiveness. Among other compounds, combustion of biomass produces highly corrosive nitrogen and sulfur compounds.
The combustion environment found in biomass stoves is unsuitable for most low-cost metals, therefore many stoves are constructed of ceramics, brick, or rock. The use of ceramic, brick, and rock, while reducing the cost of manufacture, may dramatically increase the cost of producing and distributing these stoves, decrease their durability, portability, limit combustion chamber geometry and may otherwise be undesirable.
Thus what is needed is a stove that is acceptable and accessible to persons with limited income, such as a stove that lessens the amount of toxic emissions, and may be produced from lightweight, inexpensive, corrosion-resistant materials, and that may be inexpensively and efficiently manufactured.